In battery operated (e.g. portable) devices the battery inside the device may be charged, while the device is operated, from a source (also called power source) that is power and/or current limited. The limited power from the source may be used to both operate the device and charge the battery. An example of a current and power limited source like this is a USB port providing power, another example is an AC/DC wall adapter. In this type of application it is often required that the consumer device remain operational when connected to the power source, even if the battery is removed or if it is faulty, e.g. it is shorted. Hence, an isolation of the battery from the device's circuitry (Device Circuits hereinafter) during battery charging may be desirable. This may be achieved by separating the battery from the Device Circuits when the battery is being charged from the typically external power source.
If the power source provides a substantially constant regulated voltage (as is the case in an USB power source or some AC/DC converter wall adapters for example) then the power conversion between the source voltage and the circuitry of the device is typically accomplished using high efficiency switching voltage regulators (also called switching regulators). Switching regulators may include DC/DC converters, AC/DC converters, inductor based regulators and switched capacitor based regulators. In older devices, with lower power requirements, linear voltage regulators (or linear regulators) were sometimes used. Similarly, the battery charger function may be accomplished by a separate DC/DC converter between the power source and the battery, typically controlled by a constant-current/constant-voltage (CC/CV hereinafter) dual loop controller, as it is well known in the art.
Low cost and small size may be of high importance in portable consumer devices, and a DC/DC converter is typically more expensive and requires more physical space than a linear regulator. Thus, prior art solutions envisioned cascaded DC/DC converter and linear regulator-based battery charger (linear charger hereinafter) solutions (as well as cascaded AC/DC converter and linear charger solutions) to both power the Device Circuits and charge the battery from the same power source with limited power.
A block diagram of a prior art cascaded DC/DC converter-linear charger solution is shown in FIG. 1.a. The DC/DC converter provides the supply voltage (Vsupply) for the Device Circuits and the input voltage for the linear charger. The linear charger (with its typical CC/CV controller) controls the battery charge current and ensures that the battery is not overcharged (i.e. the charging process is properly terminated when the battery is fully charged).
Typically the battery voltage changes significantly during the charging process, as the battery is charged from a fully (or partially) discharged state to a fully charged state. The battery voltage of a typical cobalt cathode Lilon battery for example can change from a minimum fully discharged voltage between typically 2.5V to 3V to a fully charged voltage (also called float voltage) of typically 4.1 or 4.2 V (depending on the anode material). As the Vsupply output voltage of the DC/DC converter is typically a substantially constant regulated voltage in the some prior art solutions and the battery voltage (Vbat) changes significantly during the charge process the dissipation on the pass device of the linear charger (which is connected between Vsupply and Vbat in FIG. 1.a) also changes significantly during the charging process.
For example if Vin=5V, Vsupply=4.4V, the battery charge current is set at Icharge=1A and the battery is a single cell Lilon battery with a fully discharged (minimum) voltage of 2.7V and a float voltage of 4.2V the dissipation of the pass device of the linear charger changes from a 1.7 W maximum to approximately zero (when the battery is fully charged and the change current is substantially zero). However, a relatively high dissipation (like 1.7 W in this example) may be difficult to accommodate in a portable consumer device (like a smart phone), which has a small physical size, without the device getting excessively hot.
One of the prior art solutions to reduce the above mentioned power dissipation problem of the linear charger, in a cascade DC/DC converter-linear charger arrangement, is making Vsupply variable instead of substantially constant. The prior art circuit disclosed in U.S. Pat. No. 7,710,079 for example regulates the DC/DC converter's output voltage (Vsupply) to track the battery voltage (Vbat) with a given offset voltage. In this prior art circuit, Vsupply can be regulated to be, for example, 200 mV higher than the battery voltage, for a certain range of battery voltages. This offset provides an approximately constant overhead voltage (input-output voltage difference, Voverhead), which is necessary for the proper operation of the linear charger. This substantially constant overhead solution would limit the dissipation of the linear charger of the above example to Pdiss=Voverhead*Icharge=200 mW during the battery charging process, representing an approximately 8× reduction in maximum dissipation compared to the previous prior art example.
A cascade combination of a DC/DC converter with one or more linear voltage regulators, whereby the output voltage of the DC/DC converter is controlled to reduce the overhead (hence the dissipation) of the linear regulators was disclosed in prior art U.S. Pat. No. 7,486,058.